The subject matter disclosed herein relates to active stack temperature management via controlled steam extracted in a combined cycle power plant.
A combined cycle power plant utilizes a gas turbine engine and a steam turbine engine in combination to produce power. The power plant is arranged such that the gas turbine engine is thermally connected to the steam turbine engine through a heat recovery steam generator (“HRSG”). The HRSG is a non-contact heat exchanger that allows feedwater for the steam generation process to be heated by otherwise wasted gas turbine engine exhaust gases. The HRSG is a large duct with tube bundles interposed therein such that water is heated to steam as the exhaust gases pass through the duct.
Modern combined cycles typically employ two (2) or three (3) steam generation pressures to recover energy from the gas turbine engine exhaust. These cycles also often include water supply temperature controls to maintain the tubes above the water dew point so as to avoid potential corrosion. Such cycles may set HRSG exit gas temperatures as low as about 150° Fahrenheit (about 66° Celsius) while, in some instances, a higher HRSG exit gas temperature is required. For example, a higher HRSG exit gas temperature may be required in accordance with local regulations to allow for a lower cost (shorter) stack but can result in an unavoidable performance reduction associated with recovery of less exhaust energy.
Higher HRSG exit gas temperatures have been previously achieved by reduction of low pressure surface area because the low pressure steam production thus sacrificed has the lowest potential to make work. By contrast, in a recent attempt to solve this problem, a low pressure steam extraction pipe and condensate feedwater heater achieve the required HRSG exhaust gas temperature across an ambient temperature range but, in this case, the hottest temperature in the ambient range tends to drive the design specifications for the extraction pipe and the condensate feedwater heater. Thus, at colder ambient temperatures, this recent arrangement would cause the HRSG exhaust gas temperature to tend to increase above the temperature requirement thus reducing thermal efficiency beyond what is required.